Rinsing a carbohydrate (CHO) solution around the oral cavity without ingestion can significantly enhance performance during running and cycling lasting ∼30–90 minutes (6,25,29). The efficacy of CHO rinsing is thought to be related to detection of CHO by oral receptors and subsequent stimulation of brain regions associated with motor control, motivation, and arousal (7,20).
Most early work investigating CHO mouth rinses focused on endurance (≥30 minutes) exercise. However, Gant et al. (14) reported significant increases in corticomotor excitability and maximal voluntary force of the elbow flexors immediately after the introduction of a CHO mouth rinse to the oral cavity. The rapid influence of a CHO mouth rinse on muscle force production suggests a potential role for this practice during shorter duration work requiring higher force output, such as resistance training. However, the data on CHO mouth rinsing and muscular strength and endurance are conflicting. Jensen et al. (19) reported significantly better maintenance of peak and average knee extensor torque during a maximal voluntary contraction after a fatiguing submaximal contraction. However, the use of a single-leg isometric protocol limits ecological validity. Using a more ecologically valid protocol, Painelli et al. (23) found no influence of a CHO mouth rinse on bench press maximum strength (1 repetition maximum [1RM]) or strength endurance (6 sets to failure at 70% 1RM). Clarke et al. (9) also examined the influence of a CHO mouth rinse on bench press maximal strength, but also utilized a more muscular endurance oriented test (repetitions to failure at 60% 1RM). There was no benefit of the CHO mouth rinse on strength or endurance. Clarke et al. (9) stated that their muscular endurance protocol elicited near maximal ratings of perceived exertion (RPE) and heart rate (HR) values, and that this “ceiling effect” may have made any potential differences between conditions hard to distinguish. The authors suggested that making the test more endurance-focused by reducing the percentage of 1RM may have revealed some of the central ergogenic effects of a CHO mouth rinse that have been reported in previous endurance-based studies.
Methodological considerations regarding the mouth rinse protocol may also influence the efficacy of mouth rinsing on muscular strength and endurance. Firstly, a dose–response relationship may exist between duration of oral exposure to a CHO mouth rinse and the efficacy of the rinse (26). Therefore, using an additional mouth rinse between the strength and endurance protocols, as suggested by Clarke et al. (9), may be more appropriate for facilitating any potential ergogenic effects of the rinse (6,7,25). Secondly, studies that have documented changes in brain imaging with CHO mouth rinses (7,28) employed a greater CHO concentration than that used in previous muscular strength and endurance research (9,23). Using a CHO concentration that has been shown to alter brain activity in ways analogous to the proposed mechanisms for enhancement of muscular strength and endurance may also potentiate the ergogenic effect of a mouth rinse during resistance exercise.
The current study was designed as an extension of previous research to further investigate the influence of CHO mouth rinsing on resistance exercise. The aim of the study was to investigate the effect of a CHO mouth rinse on upper-body maximal muscular strength and endurance. It was hypothesized that the CHO mouth rinse would significantly increase muscular strength and endurance.
Experimental Approach to the Problem
This study used a repeated measures, randomized, counterbalanced design with double blinded prescription of mouth rinses. All trials took place in the afternoon (14.00–16.00) to minimize the diurnal influence on muscle strength (8). The experimental design was based on the work of Painelli et al. (23) and Clarke et al. (9), using a bench press protocol because of the relatively untrained nature of the subjects (9). A no-rinse control condition was incorporated in line with recommendations for mouth rinse research (13). To maintain ecological validity, subjects were not requested to fast for an extended period of time before testing (4), but were requested to consume only water for the 90 minutes before testing to reduce the possibility of gastrointestinal disturbances.
Twelve recreationally resistance-trained males (mean ± SD age: 22 ± 1 year, (range 21–23 years) height: 179.2 ± 1.8 cm, body mass (BM): 80.9 ± 6.1 kg) participated. Subjects were required to have been injury free and taking part in recreational upper-body resistance training at least once a week (mean 3 ± 1 times per week) for a minimum of 6 months (9). Subjects attended 4 trials each separated by 2–7 days to limit fatigue and training effects. Subjects were asked to refrain from exercise, alcohol, and caffeine intake for 24 hours before each session, to complete a 24-hour dietary record before the first testing session, and to replicate this diet for 24 hours before each subsequent session to standardize endogenous energy content (12,22). Adherence to these procedures was verbally confirmed at the beginning of each trial. The potential risks and benefits of the protocol were explained to the subjects, after which they provided written informed consent. The study received institutional ethical approval and was conducted in line with the Declaration of Helsinki.
Subjects attended a familiarization session where anthropometric data were collected (height: Seca stadiometer; Seca, Birmingham, United Kingdom; BM: Seca scales; Seca, Birmingham, United Kingdom). The full protocol was then undertaken using plain water as the mouth rinse.
The 1RM and repetitions to failure protocols were conducted on a bench press rack with safety bars in place (Power Lift, Iowa, USA) and in the presence of a qualified spotter. Strong verbal encouragement was provided for all maximal lifts and during the repetitions to exhaustion test. Subjects' 1RM was assessed using the protocol of Earle and Beachle (11) as described by Clarke et al. (9). The subject warmed up by performing 10 repetitions with a 20 kg bar (Eleiko; Eleiko AB, Halmstad, Sweden) followed by 1-minute rest. The weight (Eleiko Olympic disks; Eleiko AB, Halmstad, Sweden) was then increased by 10% and the subject performed 3–5 repetitions. After a 2-minute rest, a weight near maximum was chosen by the subject, based on past training experience, and was lifted for 2–3 repetitions. The subject then rested for 3 minutes, the load was increased by 5–10% based on researcher and subject perceptions (9), and the subject performed their first 1RM attempt. If successful, the load was increased by 5–10% and attempted again after a 3-minute rest. If unsuccessful, the load was decreased by 2.5–5% for another attempt after a 3-minute rest. This process was repeated for a maximum of 5 attempts until a 1RM was identified. One repetition was defined as lowering the bar, so it touched the chest then raising the bar until elbows were fully extended. Bar grip position was recorded at each subjects' familiarization session and replicated for subsequent sessions. Subjects were instructed to keep their buttocks on the bench and their heels touching the floor for every repetition to standardize lifting technique.
After determination of 1RM, the subject rested for 1 minute to allow the weight to be adjusted to 40% 1RM (9). They then performed repetitions to failure at this load (9). Repetitions to failure was defined as the maximum number of unassisted repetitions using correct technique that participants could carry out before volitionally terminating the test. Total exercise volume (kg) was calculated by multiplying 40% of the subjects' 1RM by the number of repetitions completed.
Rating of perceived exertion (RPE) (5) was recorded immediately after the repetitions to exhaustion. Heart rate (Polar FS1 heart rate monitor; Polar Electro, Warwick, United Kingdom) was recorded immediately before each mouth rinse and immediately after each exercise protocol. Felt arousal (FA) (27) was recorded immediately before and after both mouth rinses (9). The FA scale measures arousal levels on a scale ranging from 1 (low arousal, including sensations such as relaxation, boredom, or calmness) to 6 (high arousal, including sensations such as excitement, anxiety, and anger) (27).
Subjects carried out 3 trials using the same procedures described above. Two mouth rinses were used: An 18% maltodextrin (Bulk Powders maltodextrin; Bulk Powders TM, Colchester, United Kingdom) solution (CHO) (7) and a water placebo solution (PLA). A no-rinse control trial was also used (CON). A commercially available electrolyte tablet (HighFive, Bardon, Leicestershire) was dissolved into each solution, providing the following electrolyte profile per liter: sodium 250 mg, magnesium 60 mg, potassium 90 mg, calcium 20 mg. The tablet contained a small amount of artificial sweetener (Saccharine) and was citrus flavored. Previous research has demonstrated that the electrolyte tablets are effective blinding agents (24), and pilot testing confirmed this for the higher CHO concentration used in the current study. An individual unrelated to the study coded and distributed the mouth rinses, and the nature of the coding was only revealed after data collection was completed. Subjects swilled a 25 ml bolus (6,24) around their oral cavity for 10 seconds (26) before expectorating it into a plastic container before the first 1RM attempt (9) and repetitions to failure. There was a 10-second gap between expectorating the mouth rinse and beginning the exercise.
Data are reported as mean ± SD, unless specified. Statistical analyses were conducted using IBM SPSS Statistics for Windows, version 21 (IBM Corp, Armonk, NY, USA). The Shapiro–Wilk test assessed the distribution of all data sets. A 1-way analysis of variance (ANOVA) with repeated measures compared order and treatment main effects for 1RM, repetitions to failure, total exercise volume, RPE, and FA, and treatment main effects for RPE. Unstandardized mean differences with 95% confidence intervals (95% CI) between the CHO and PLA trials and the CON trial were also calculated for 1RM, repetitions to failure, and total exercise volume. Heart rate and FA were analyzed using 2-way (trial × time) ANOVA with repeated measures. Mauchly's test analyzed the sphericity assumption and the Greenhouse–Geisser adjustment was used when required. Significant main effects were explored using pairwise comparisons with the Bonferroni correction applied. An alpha of p ≤ 0.05 was the threshold for statistical significance. Effect sizes for ANOVA main effects were presented using partial eta-squared (). For pairwise comparisons, Cohen's d effect sizes for within-subjects designs (21) were calculated and defined as trivial (d < 0.2), small (d ≥ 0.2, <0.8), medium (≥0.5, <0.8), and large (d ≥ 0.8) (10).
No significant order effects were found for 1RM (F2,22 = 2.424; p = 0.112, = 0.18), repetitions to failure (F2,22 = 1.047; p = 0.368, = 0.09), or total exercise volume (F1.3,13.8 = 2.871; p = 0.107, = 0.21). Similarly, there were no order effects for RPE (F2,22 = 1.526; p = 0.240, = 0.12) or FA (F2,22 = 1.000; p = 0.384, = 0.08).
No significant differences in 1RM were found between trials (F2,22 = 0.393; p = 0.680, = 0.03; CHO vs. PLA: d = 0.21; CHO vs. CON: d = 0.13; PLA vs. CON: d = 0.16; Figure 1A). Figure 1B displays the mean (±95% CI) difference in 1RM in the CHO and PLA trials vs. CON. Although the mean change in the CHO trial was positive, the CI for the CHO and PLA trial was large and included the null value (zero change).
There were no significant differences between trials for repetitions to failure (F2,22 = 0.397; p = 0.677, = 0.04; CHO vs. PLA: d = 0.21; CHO vs. CON: d = 0.22; PLA vs. CON: d = 0.07; Figure 1C). Figure 1D displays the mean (±95% CI) difference in 1RM in the CHO and PLA trials vs. CON. The difference in repetitions to exhaustion was positive for both trials; however, as with the 1RM data, the CIs were large and included the null value (zero change).
There were no significant differences between trials for total exercise volume (F2,22 = 0.523; p = 0.600, = 0.05; CHO vs. PLA: d = 0.30; CHO vs. CON: d = 0.22; PLA vs. CON: d = 0.01, Figure 1E). Figure 1F displays the mean (±95% CI) difference in total exercise volume in the CHO and PLA trials vs. CON. The difference in total exercise volume was positive for the CHO trial; however, the CI for the CHO and PLA trials was large and included the null value (zero change).
Heart rate (Table 1) did not differ significantly between trials (F2,22 = 0.396; p = 0.677, = 0.04). There was a significant main effect of time (F1.7,19.0 = 213.669; p < 0.001, = 0.95), with HR significantly increasing from pre-1RM testing to post-1RM testing (p < 0.001, d = 4.10) and from prerepetitions to postrepetitions to failure (p < 0.001, d = 4.87). There was no significant trial × time interaction (F2.6,28.6 = 2.071; p = 0.133, = 0.16). Felt arousal (Table 1) did not differ significantly between trials (F2,22 = 0.401; p = 0.674, = 0.04). There was a significant main effect of time (F3,33 = 237.239; p < 0.001, = 0.96), with FA significantly increasing presecond rinse compared with postfirst rinse (p < 0.001, d = 5.23). There was no significant trial × time interaction (F2.8,30.6 = 0.880; p = 0.455, = 0.07). Rating of perceived exertion did not differ significantly between trials (CHO 17 ± 1, PLA 17 ± 1, CON 17 ± 0, F2,22 = 0.517, p = 0.604, = 0.05).
The main finding of the current study is that a CHO mouth rinse does not increase maximum muscular strength or endurance in recreationally resistance-trained subjects. Both study hypotheses are therefore rejected.
There may be a dose response relationship between duration of oral exposure to a CHO mouth rinse and its efficacy (26). The current study incorporated an additional mouth rinse before the repetitions to exhaustion test, in contrast to Clarke et al. (9). However, this did not potentiate the effect of the mouth rinse. This finding is in line with Painelli et al. (23), who found no significant improvement in muscular strength or strength-endurance with multiple administrations of a CHO mouth rinse. Any effects of a CHO mouth rinse on muscle function may be short-lived (19); therefore the time between administrations should be considered. In the current study, the time between the two CHO mouth rinse administrations may have been too long for a cumulative effect to be seen.
Painelli et al. (23) fasted their subjects overnight before testing, whereas the current study and Clarke et al. (9) did not. Using a CHO mouth rinse in a fasted state may potentiate its ergogenic effect, at least on endurance performance (4,12). Based on the different methodological approaches of Painelli et al. (23) and Clarke et al. (9), it appears that the effect of a CHO mouth rinse on resistance exercise performance is not influenced by subjects' dietary status. However, the influence of dietary status as the sole independent variable on muscular strength and endurance remains to be fully elucidated.
Foods with a higher energy density activate more brain regions than foods with a lower energy density (7,15,17). Studies that have demonstrated significant increases in activity of brain regions associated with motor control, motivation, and arousal with CHO mouth rinses used CHO concentrations of 15–18% (7,28). These concentrations are notably higher than the majority of performance research that has employed concentrations of ∼6% (6,9,23,24). The current study used an 18% concentration mouth rinse. Although brain imaging was not possible in this study, the lack of effect of an 18% CHO mouth rinse on muscular strength or endurance provides further evidence that CHO mouth rinsing is not beneficial for improving these parameters. Although it is possible that a specific combination of mouth rinse composition and administration frequency could elicit muscular strength or endurance enhancements, the available literature suggests that CHO mouth rinsing is not a practical strategy for enhancing this form of exercise.
Clarke et al. (9) suggested that the lack of influence of a CHO mouth rinse on muscular endurance in their study may have been because of the attainment of near-maximal HR and RPE values creating a “ceiling-effect”, making it difficult to observe appreciable differences between conditions. For this reason, Clarke et al. (9) recommended that future studies use a lower percentage of 1RM in the endurance test, as the current study did. The current study reported almost identical RPE values to Clarke et al. (9), and similar HR (with the exception of the CHO trial, which was ∼15 b·min−1 lower in the current study). Therefore, the use of a lower load does not enable performance enhancing effects of a CHO mouth rinse to be seen. It also questions the “ceiling-effect” suggestion of Clarke et al. (9). The highest mean HR value recorded by Clarke et al. (9) and the current study at the end of the endurance test was ∼77% of age-predicted maximum HR, and the RPE values were ∼17. These values, particularly for HR, are not “near maximal”. Furthermore, values similar to or notably higher than these have been reported in previous studies that found significant improvements in performance with a CHO mouth rinse, albeit in different exercise modalities (3,6,18,24). It therefore appears that the centrally-mediated improvement in exercise performance sometimes observed with a CHO mouth rinse is not evident during resistance exercise of the type used in this study (9,23). This statement is further supported by the lack of influence of CHO mouth rinsing on FA, with the significant main effect of time on FA likely due to the performance of the 1RM test (9), as the test took place between the postfirst rinse and presecond rinse measures of FA, and FA increased to the same extent across all 3 trials.
Strength-trained people can produce greater neuromuscular activation than non–strength-trained people (2). This has led to the suggestion that the stimulation of brain regions associated with motivation and motor control by CHO mouth rinses is insufficient to affect strength performance in strength-trained individuals, but may elicit improvements in non-strength-trained subjects (23). However, the rationale for non-strength-trained individuals using a CHO mouth rinse to enhance strength is questionable when it is considered that much of the initial increase in muscular strength after the onset of a resistance exercise program is attributable to neural adaptations (1). Furthermore, subjects in the studies of Clarke et al. (9), Painelli et al. (23), and the current study could not be classed as strength trained because the approximate relative strength (1RM/BM) of the subjects places them around the 60th-70th percentile based on general population normative data (16). Therefore, any potential influence of a CHO mouth rinse in less well-resistance-trained subjects could have been expected to manifest across these 3 studies.
A limitation of the current study is the use of a bench press protocol because this is not an activity that will be routinely used in the training of most athletes. However, the bench press protocol provided a balance between ecological validity, particularly compared with protocols using isometric single leg dynamometry (19), and retaining experimental control. Future work should enhance the ecological validity of the muscular strength and endurance protocols. The fixed load nature of the protocol was also a potential limitation because it did not allow the subjects to self-select loads. Employing a self-managed resistance training protocol more analogous to a normal training scenario would elucidate whether CHO mouth rinsing allows the selection of higher loads for a given intensity/repetition target, similar to the self-selection of higher power output observed when CHO rinsing during endurance exercise (6,25,29). Finally, subjects were requested to replicate their dietary intake for 24 hours before each trial, but dietary intake was not analyzed to confirm standardization of macronutrient consumption before the 3 trials. It would have been useful to analyze dietary composition to control for the potentially confounding factor of endogenous energy availability on CHO rinse efficacy.
In conclusion, a CHO mouth rinse does not significantly affect maximal muscular strength or endurance. Carbohydrate mouth rinsing may not provide a sufficient central stimulus to improve resistance exercise performance under the conditions and with the subjects used in the study.
The data from the current study found no significant or practically meaningful improvement or detriment in muscular strength and endurance with the use of a CHO mouth rinse compared with a PLA or CON. These data suggest that athletes and coaches should not employ a CHO mouth rinse to enhance upper-body maximal muscular strength or endurance. Research has not focused on upper-body exercises other than the bench press, ecologically valid lower-body exercises, or differently structured resistance training sessions; therefore, practical applications cannot currently extend to these scenarios.
None of the authors have any professional relationships with companies or manufacturers who will benefit from the results of the present study. None of the authors have any conflicts of interested associated with this study. No grant support or other funding was received for this project. The results of the present study do not constitute endorsement of the product by the authors or the NSCA.
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